Algae in Koi Ponds & Water Gardens

There are 2 main types of pond algae that affect Koi Ponds and Water Gardens. We have broken them down into the following groups:

  • Planktonic (AKA Green Water Algae)
  • Filamentous (AKA String, Blanket or Hair Algae)


Planktonic Pond Algae

This is the type of pond algae that causes green water. Certain types of Blue-green algae can even be toxic. Generally, green water is either an indication of a pond with inadequate or mismanaged filtration, or it is a result of a nutrient surplus.

Koi Pond and Water Garden Owners can do many things to combat green water, but there are 4 key steps that should always be considered.

  1. Make sure that water does not run off from your lawn into your pond when it rains and that no soil or other organic substances are getting into your pond. These are primary sources of nutrients that fuel algae growth.
  2. Confirm that you have an adequately sized biological filter system and the proper water flow through your pond. The entire volume of pond water should be able to pass through your filter at least of once every 2 hours.  Once an hour is much preferred. Also, make sure you do not overly clean the biological media portion of your biofilter.  This can kill the beneficial bacteria.
  3. Have patience with new ponds and let them develop a base of beneficial bacteria to filter the water. This process should be initially accelerated with the application of Pond Bacteria. Also, before shutting your pumps down for the winter, rinse your Biological Filter Media with pond water (so you don`t temperature shock it) and put it in your pond for the winter, not your garage! This way when spring comes around, your filter will be alive and ready to go with instant filtration!
  4. Add Aquatic Plants to take up nutrients and block some of the sunlight from reaching the water. Consider planting submerged plants (a.k.a. oxygenators), water lilies and/or floating plants to cover up to 1/2 of the pond surface. Please note that this fourth step can be difficult to achieve in ponds with high density Koi populations. Some pond owners segregate their pond with plants in one half and Koi in the other.  Also, if soil from plant pots gets loose into the water it will add to the nutrient problem. 

Once you have done all of the above, the following additional methods can be used to clear the water:

  • Aerate the pond if you have stagnant sections, or if you do not have a vigorous waterfall that runs 24/7.
  • Apply Pond Dye to tint the water, which limits how far sun can penetrate into the water.
  • Apply a Phosphorous Binder/Remover to remove nutrients from the water and help clump up and remove suspended particulate matter.
  • Barley Products, including Barley Straw & Barley Extract can be used as an effective natural treatment to clean the water.
  • Water Clarifiers can be used to clump suspended particulate matter together, and then let the filter remove it from the water.
  • Add a UV Clarifier to kill algae cells as they pass through the system. Be sure not to apply bacteria while the clarifier is active, as it will kill them as well. Note that UV Clarifiers have no affect on Filamentous Algae.

Please note we generally do not recommend the use of Algaecides against planktonic algae, because if you kill it all at once, that can crash oxygen levels, which can kill fish.

Filamentous Pond Algae

This is the infamous string or blanket algae. Many different types occur and some of them are resistant to algaecides. Therefore, the best approach is prevention. Here again, as discussed in the planktonic algae section above, the best thing you can do is to find the source of the nutrients and cut them off. However, no matter how diligent you are at stemming off nutrients, there is usually a certain amount of pond algae that still grows.

Koi Pond and Water Garden Owners should begin by reviewing all the tips outlined above in the Planktonic Algae section. It is critical to always keep a clean pond to prevent algae, because uneaten fish pellets and other organic matter gives off nutrients as it decomposes, which causes algae. In addition to the methods outlined above try the following:

More Info on Pond Algae

On our Algae Mngt Plan for Koi Ponds and Water Gardens page we have listed treatment programs for different pond algae problem scenarios that commonly face Koi Pond & Water Garden owners.